Preparation method of palladium alloy composite membrane for hydrogen separation

ABSTRACT

Disclosed herein is a method of preparing a palladium alloy composite membrane for hydrogen separation, including (a) providing a first metal coating layer on a porous support using an electroplating process; (b) providing a palladium coating layer on the first metal coating layer using a dry plating process; and (c) heat treating the palladium coating layer to form an alloy layer of palladium and the first metal.

This is a divisional of application Ser. No. 11/662,432, filed Aug. 30,2007.

TECHNICAL FIELD

The present invention relates, generally, to a method of preparing apalladium alloy composite membrane for hydrogen separation, and, moreparticularly, to a method of preparing a palladium alloy compositemembrane for hydrogen separation, which is advantageous becausepalladium is used in a small amount, and thus a membrane for hydrogenseparation having outstanding selectivity to hydrogen gas and highdurability can be prepared, and, as well, the properties of theseparation membrane can be improved, regardless of the kind of support.

BACKGROUND ART

Generally, a separation membrane used for the preparation of ultrahighly pure hydrogen has low permeability. Hence, in order to overcomesuch a problem, intensive and extensive research on improvement of theselective permeability of the membrane by applying a non-porouspalladium membrane on a porous support is presently being studied. Thenon-porous palladium membrane has high hydrogen selectivity but has lowpermeability. Therefore, although the selective hydrogen permeability ofthe separation membrane is intended to increase by coating the surfaceof the porous support with a thin palladium membrane, the separationmembrane coated with only palladium suffers because it may be deformeddue to phase change of the lattice structure while hydrogen gas isabsorbed. With the goal of preventing such deformation, a palladiumalloy separation membrane is mainly used at present.

A metal, which is alloyed with palladium, includes, for example, silver,nickel, copper, ruthenium, molybdenum, etc. In particular, apalladium-copper alloy membrane, which is prepared using inexpensivecopper, has resistance to hydrogen sulfide and sulfur compound poisoningsuperior to other palladium alloy membranes, and thus has beenthoroughly studied in recent years. In such cases, the alloy membrane istypically prepared by alloying a copper plating layer and a platedpalladium layer (or a sputtered palladium layer) sequentially coated ona porous ceramic support or a porous metal support. However, thepalladium-copper alloy membrane prepared using such a conventionalmethod is disadvantageous because it is not dense and has fine pores ordefects therein, thus having low hydrogen selectivity (FIG. 1). Further,when the copper layer, serving as an alloy source, is present as anintermediate layer between the support and the palladium layer, it maybe separated due to the thermal diffusion and fluid reflow properties ata usage temperature of 500° C., therefore negatively affecting theadhesion. Consequently, the palladium-copper alloy separation membranebreaks.

Turning now to FIG. 2, there is illustrated a palladium-copper alloymembrane, which comprises a palladium-copper alloy coating layerprovided on a porous metal support by sequentially forming a nickelplating layer as an underlayer of a copper plating layer, a copperplating layer and a palladium plating layer on the porous metal supportand then heat treating them. In addition, this drawing shows the resultof heat treatment for the alloy membrane at a usage temperature of 500°C. for 100 hr.

From the surface microstructure of the separated upper portion of thealloy membrane and the EDS result shown in FIG. 2, it can be seen thatthe microstructure of membrane is not dense and copper and palladium arepresent in the this portion.

FIG. 3 illustrates the surface microstructure of the separated lowerportion of the alloy membrane and the EDS result, in which themicrostructure of membrane is not dense and copper and nickel arepresent in the this portion. Thereby, it appears that the copper platinglayer is separated through the thermal diffusion of copper atoms andmoved to the upper layer (palladium coating layer) and the lower layer(support) of the copper plating layer.

Recently, a palladium alloy composite separation membrane has beendeveloped using a porous metal support made of stainless steel throughan electroplating process. However, since the pore size of the porousstainless steel support used is large and the surface thereof is rough,a complicated pretreatment procedure is required to apply the palladiumalloy separation membrane. In the case where the electroplating processis conducted on the porous stainless steel support to form a palladiumalloy coating layer, the support may be corroded by hydrochloric acidacting as a main component for activation of a plating process, andhydrogen separation properties may decrease due to additive impuritiesin a plating solution. In addition, the palladium metal is diffused intothe support at a usage temperature of 500° C., thus decreasingdurability. As well, upon reforming of hydrogen gas, hydrogenbrittleness of a stainless steel substrate is caused by the hydrogenabsorption, and thus the substrate may break.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made to overcome the aboveproblems occurring in the related art, and an object of the presentinvention is to provide a method of preparing a palladium alloycomposite membrane for hydrogen separation, which is advantageousbecause a small amount of palladium is used, thus a membrane forhydrogen separation having excellent hydrogen selectivity and highdurability can be prepared, and as well, the properties of theseparation membrane can be improved, regardless of the kind of support.

Technical Solution

According to a first embodiment of the present invention for achievingthe above object, a method of preparing a palladium alloy compositemembrane for hydrogen separation is provided, comprising (a) forming apalladium coating layer on a porous support; (b) forming a metal coatinglayer on the palladium coating layer; and (c) subjecting the metalcoating layer to a reflow process to form an alloy layer with a voidfree and dense film.

According to a second embodiment of the present invention, a method ofpreparing a palladium alloy composite membrane for hydrogen separationis provided, comprising (a) forming a first metal coating layer on aporous support using an electroplating process; (b) forming a palladiumcoating layer on the first metal coating layer; (c) forming a secondmetal coating layer on the palladium coating layer; and (d) subjectingthe second metal coating layer to a reflow process to form an alloylayer with a void free and dense film.

In the method of the present invention, the porous support is preferablya porous metal support or a porous ceramic support.

In the method of the present invention, the porous support is preferablya porous nickel support.

In the method according to the second embodiment of the presentinvention, (a) preferably further comprises heat treating the firstmetal coating layer formed using the electroplating process to removeimpurities.

In the method according to the second embodiment of the presentinvention, the first metal coating layer is preferably formed of atleast one metal selected from the group consisting of nickel, copper,and silver.

In the method according to the first embodiment of the presentinvention, the first metal coating layer is formed of nickel.

In the method according to the second embodiment of the presentinvention, the second metal coating layer is formed of copper.

Advantageous Effects

The present invention provides a method of preparing a palladium alloycomposite membrane for hydrogen separation. According to the method ofthe present invention, even though palladium is used in a small amount,the separation membrane having excellent hydrogen selectivity and highdurability can be prepared. Further, the properties of the hydrogenseparation membrane can be improved, regardless of the kind of support.

DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron micrograph showing the microstructure of apalladium-copper alloy composite membrane prepared using a conventionalmethod;

FIG. 2 is a scanning electron micrograph showing the microstructure ofthe separated upper portion of the alloy composite membrane, in which apalladium-copper alloy coating layer is provided on a porous metalsupport through heat treatment in order to assay the durability of themembrane, and showing the result of EDS analysis thereof;

FIG. 3 is a scanning electron micrograph showing the microstructure ofthe separated lower portion of the alloy composite membrane, and showingthe result of EDS analysis thereof;

FIG. 4 is a scanning electron micrograph showing the microstructure of apalladium-copper alloy composite membrane prepared through simple reflowheat treatment of a copper layer, according to the present invention;

FIG. 5 shows the result of XRD analysis for the palladium-copper alloycomposite membrane of the present invention;

FIG. 6 is a scanning electron micrograph showing the surfacemicrostructure of the alloy composite membrane, in which a nickelcoating layer, a sputtered palladium coating layer and a sputteredcopper coating layer are sequentially formed on a porous nickel supportand then heat treated at 600° C. higher than an actual usage temperaturefor 20 days in a nitrogen atmosphere, in order to observe thermalstability;

FIG. 7 shows the result of crystal structure analysis of thepalladium-copper-nickel alloy composite membrane of the presentinvention;

FIG. 8 is a scanning electron micrograph showing the cross section of analloy composite membrane sample of the present invention;

FIG. 9 is an EDS line scan showing the cross section of the alloycomposite membrane sample of the present invention;

FIG. 10 is a scanning electron micrograph showing the surfacemicrostructure of a palladium-copper alloy composite membrane, resultingfrom reflow heat treatment of a palladium coating layer and a coppercoating layer, each of which is formed using an electroplating process,according to the present invention;

FIG. 11 is a scanning electron micrograph showing the surfacemicrostructure of a palladium-copper alloy composite membrane formed ona porous alumina support using a reflow process, according to thepresent invention; and

FIG. 12 shows the hydrogen/nitrogen separation of the palladium-copperalloy composite membrane formed on a porous nickel support using acopper reflow process, upon use of a gas mixture including hydrogen andnitrogen, according to the present invention.

BEST MODE

Hereinafter, a detailed description will be given of a method ofpreparing a palladium alloy composite membrane for hydrogen separation,according to a first embodiment of the present invention.

Based on the present invention, as the support for the compositemembrane, a porous metal support or a porous ceramic support may beused. The porous support may be a planar type or a tubular type.Compared to porous ceramic supports, the porous metal support isadvantageous because it entails a lower preparation cost, higher thermalimpact resistance and mechanical strength, and higher processability andmodularity, and is thus suitable for application in highly pure hydrogenseparation and purification systems or catalyst reactors.

In particular, a porous nickel support has good chemical affinity topalladium and nickel, which are main components of the palladium alloycomposite membrane. Further, compared to porous stainless steel metalsupports, the porous nickel support does not generate hydrogenbrittleness due to intrinsic properties thereof, and has higherresistance to corrosion by hydrochloric acid. The porous nickel supportresulting from sintering of nickel powder has an average pore size ofsub-μm or less than, and the pore density thereof is uniform, thus nocomplicated pretreatment is required when coated with a palladium alloycomposite membrane. In addition, the porous nickel support itself hashydrogen selectivity of about 8˜10 and permeability of 150ml/cm²·atm·min or more and thereby has properties suitable for use inthe metal support of a palladium alloy composite membrane.

A palladium coating layer, which is applied on the porous support, maybe formed using either a wet electroplating process or a dry sputteringdeposition process. Preferably, an electroplating process is adopted, sothat the surface pores of the porous support are completely filled andthe surface flatness thereof is attained. In this case, with the aim ofimproving adhesion between the palladium coating layer and support, itis preferable that the surface of the porous support be modified usingplasma surface treatment before the formation of the palladium coatinglayer. Specifically, the plasma condition for surface modification mayvary with the process and is not particularly limited. For example, inthe case of using a porous nickel support, the plasma process may beperformed under conditions of RF 100 W, 50 mTorr, with an amount ofhydrogen of 40 sccm, and for 5 min.

When an electroplating process is used to form the palladium coatinglayer, it is not particularly limited, and may preferably be performedunder conditions of a current density of 10 mA/dm², a plating time of 20min, and a plating bath temperature of 40° C. Alternatively, in the caseof using a dry sputtering deposition process, this process does not needa particular limitation, and may preferably be carried out underconditions of direct current (DC) power of 40 W, an amount of argon gasof 25 sccm, a process pressure of 1.0×10⁻³ torr and a substratetemperature of 400° C.

Subsequently, a metal coating layer is formed on the palladium coatinglayer. The kind of metal constituting the metal coating layer is notparticularly limited, and includes, for example, silver, nickel, copper,ruthenium, or molybdenum. Of these metals, copper is preferably used,because it is economically advantageous and has high resistance tohydrogen sulfide and sulfur compound poisoning for the alloy membranewith palladium.

The metal coating layer may be formed using either a wet electroplatingprocess or a dry sputtering deposition process. In the case where thepalladium coating layer is applied using an electroplating process, themetal coating layer may be electroplated through a continuous process,or be formed using sputtering deposition. In addition, in the case wherethe palladium coating layer is formed using a dry sputtering depositionprocess, the metal coating layer may be deposited by sputtering througha continuous process, or may be formed using electroplating.

The electroplating procedure of the metal coating layer by a continuousprocess may be carried out under slightly different conditions,depending on the kind of metal and an process ambience. For example, inthe case where copper is used as a metal component, it may be coatedusing a copper cyanide plating solution under conditions of a currentdensity of 200 mA/dm², a plating time of 30 sec and a plating bathtemperature of 40° C.

In addition, when the metal coating layer is formed using sputteringdeposition by a continuous process, copper may be deposited underconditions of DC power of 30 W, an amount of argon gas of 20 sccm, aprocess pressure of 1.0×10⁻³ torr, and a substrate temperature of 400°C.

The two-layer metal membrane thus obtained is alloyed by a subsequentreflow process, to form a palladium-metal alloy composite membrane. Thereflow process is preferably performed in an in-situ process, and may becarried out through heat treatment at 500˜700° C. in a vacuum of 1 mTorrin a hydrogen atmosphere in a vacuum heating furnace. The reflow processresults in a uniform composite membrane having a dense alloy structurewithout defects or fine pores.

In addition, a method of preparing a palladium alloy composite membranefor hydrogen separation, according to a second embodiment of the presentinvention, is specifically described below.

The method according to the second embodiment is the same as thepreparation method of the first embodiment, with the exception offurther including a process of forming a metal underlayer (first metalcoating layer) on a porous support, before forming a palladium coatinglayer, to fill the surface pores of the porous support. Preferably, themetal underlayer is a nickel metal layer, which may be formed usingelectroplating. The electroplating process is preferable to a drysputtering deposition process, because it enables the surface pores ofthe porous support to be completely filled and causes the surface to beflat. Further, in order to improve adhesion before the formation of thenickel plating layer, the surface modification of the porous supportusing plasma is preferably performed. The advantage thereof is the sameas mentioned in the first embodiment.

Then, subsequent processes of forming a palladium coating layer and asecond metal coating layer (corresponding to the metal coating layer inthe first embodiment) are conducted in the same manner as in the firstembodiment, and the detailed description thereof is omitted.

Before the formation of the palladium coating layer, the surface of thefirst metal coating layer is preferably modified using plasma. Theplasma treatment conditions are the same as those of the porous support.

Below, the present invention is explained based on an alloy compositemembrane for hydrogen separation using a nickel metal layer as a firstmetal coating layer on a porous nickel support and a copper metal layeras a second metal layer on the palladium coating.

FIG. 4 is a scanning electron micrograph showing the microstructure ofthe composite membrane according to the first embodiment of the presentinvention, and FIG. 5 shows the result of XRD analysis of the alloycomposite membrane. From the surface microstructure of FIG. 4 and thecrystal analysis of FIG. 5, the palladium-copper alloy membrane formedon the porous nickel support can be confirmed to be a uniform separationmembrane, which has a dense structure, without defects or fine pores.

FIG. 6 shows the result of thermal stability of the alloy compositemembrane according to the second embodiment of the present invention. Toobserve such thermal stability, a 3 μm thick nickel coating layer, a 4μm sputtered palladium coating layer, and a 1 μm sputtered coppercoating layer are sequentially formed on a porous nickel support andthen heat treated at 600° C. higher than an actual usage temperature for20 days in a nitrogen atmosphere. Such heat treatment corresponds tothermal effect similar to heat treatment at 500° C. for 1 year orlonger, assuming that the other diffusion conditions thereof are thesame.

FIG. 6 is a scanning electron micrograph showing the surfacemicrostructure of the membrane. As shown in the drawing, the alloy layeris dense, without defects or fine pores, even though heat treatment isperformed. As is apparent from the XRD analysis of FIG. 7, throughcontinuous heat treatment, the chemical affinities of palladium, copperand nickel for each other are good, and thus, a stable ternary alloymembrane of palladium-copper-nickel is formed and strongly adheres onthe support.

FIG. 8 shows the result of diffusion in metals, in which palladium isdiffused into the support. As seen in an EDS concentration distributionof FIG. 9 through heat treatment, the palladium metal is diffused intothe porous support, but a great amount of palladium is still present inthe coating layer, thus forming a palladium-copper-nickel alloyseparation membrane. As can be shown in the cross section of themicrostructure of FIG. 8, the alloy membrane is dense, and also,adhesion between Pd alloy membrane and support is excellent to theextent that the boundary between the support and the alloy coatingmembrane is not observed even using a scanning electron microscope.

From the results of FIGS. 8 and 9, the palladium metal is less diffusedinto the porous metal support despite the use of the porous metalsupport, and thus, a great amount of palladium is present in the form ofpalladium-copper-nickel alloy composite membrane. The structure of thealloy layer is still dense, and the thermal stability of thepalladium-copper-nickel alloy composite membrane is excellent, wherebythe durability thereof is considered to be improved compared toconventional results.

FIG. 10 is a photograph showing the microstructure of a palladium-copperalloy composite membrane, resulting from reflow heat treatment of apalladium coating layer and a copper coating layer, each of which isformed using electroplating. As shown in this drawing, the alloymembrane is dense and has no fine pores.

FIG. 11 is a photograph showing the microstructure of a palladium-copperalloy composite membrane formed on a porous alumina support using areflow technique, in which the alloy membrane is confirmed to have avery dense structure without fine pores.

FIG. 12 shows the hydrogen/nitrogen selectivity varying with the usagetemperature of the palladium-copper alloy composite membrane of thepresent invention, when using a gas mixture including hydrogen andnitrogen under 2.2 psi pressure. As shown in the drawing, as thetemperature increases, the selectivity increases and then reaches aninfinite value at 500° C., thus exhibiting excellent properties.

Compared to the hydrogen/nitrogen selectivity of a palladium-copperalloy separation membrane prepared using a conventional technique shownin Table 1 below, the palladium-copper alloy composite membrane of thepresent invention is confirmed to have higher hydrogen selectivity.

TABLE 1 Tem- Permea- Selec- Thick- Δ P perature bility (ml/ tivity ness(kPa) (K) cm² · min) (H₂/N₂) (μm) Gas Ex- 100 773 9 ∞   3 ± 0.1 A gasample mixture including H₂ and N₂ 1¹⁾ 689.5 723 6.45 14 27.6 ± 8.5 Eachseparated H₂ and N₂ gas 2¹⁾ 344.7 973 47 70 11.0 ± 1.0 ″ 3¹⁾ 344.7 77369.9 170 11.6 ± 1.0 ″ 4¹⁾ 344.7 723 24 270 12.5 ± 1.5 ″ 5¹⁾ 344.7 723107 1400   12 ± 1.0 ″ 6¹⁾ 344.7 723 88 47  1.5 ± 0.2 ″ Note: ¹⁾FernandoRoa, Douglas Way, Robert L. McCormick, Stephen N. Paglieri “Preparationand characterization of Pd—Cu composite membranes for hydrogenseparation” Chemical Engineering Journals. 93 (2003)11-22.

MODE FOR INVENTION

A better understanding of the present invention may be obtained in lightof the following example which is set forth to illustrate, but is not tobe construed to limit the present invention.

Example

A porous nickel support was surface treated using hydrogen plasma. Thesurface treatment using hydrogen plasma was carried out under conditionsof RF power of 100 W, an amount of hydrogen of 40 sccm, a processpressure of 50 mTorr, and a period of time of 5 min. Subsequently, inorder to fill the surface pores of the support, a nickel electroplatingprocess was performed on the surface treated support at room temperatureand a current density of 1 A/dm² for a plating time of 2 min using anickel chloride plating solution. After the nickel electroplatingprocess, the support was dried in a vacuum drying oven at 60° C., andthen maintained at 200° C. in a vacuum atmosphere of 10⁻³ torr for 1 hrto remove dust and impurities from the inside of the support.

The support was further subjected to hydrogen plasma treatment and thenpalladium electroplating using a palladium chloride solution underconditions of a current density of 10 mA/dm², a plating time of 20 min,and a plating bath temperature of 40° C. Thereafter, a copperelectroplating process was coated using a copper cyanide solution by acontinuous process, under conditions of a current density of 200 mA/dm²,a plating time of sec, and a plating bath temperature of 40° C. Afterthe formation of coating layers, reflow heat treatment was carried outat 700° C. in a vacuum of 1 mTorr in a hydrogen atmosphere for 1 hr,thus alloying a copper layer and a palladium layer.

On the other hand, in the case where a sputtering process was employedas a dry preparation method, instead of the above wet preparationmethod, all pretreatment processes and the formation process of a nickelcoating layer as a first coating layer were the same as in the above wetmethod, and only the sputtering process was carried out differently, asfollows.

For the deposition of palladium, sputtering was carried out underconditions of a DC power of 40 W, an amount of argon gas of 25 sccm, aprocess pressure of 1.0×10⁻³ torr, and a substrate temperature of 400°C., and copper sputtering was continuously deposited under conditions ofDC power of 30 W, an amount of argon gas of 20 sccm, and a processpressure of 3.0×10⁻³ torr and a substrate temperature of 400° C.Subsequently, an in-situ reflow process was performed under heattreatment conditions of a vacuum of 1 mTorr, and a reflow temperature of700° C. for 1 hr in a hydrogen atmosphere of a vacuum heating furnace,thus obtaining a palladium-copper-nickel alloy composite separationmembrane.

INDUSTRIAL APPLICABILITY

As described hereinbefore, the present invention provides a method ofpreparing a palladium alloy composite membrane for hydrogen separation.According to the method of the present invention, even though palladiumis used in a small amount, a hydrogen separation membrane havingoutstanding hydrogen selectivity and high durability can be prepared.Further, the properties of the hydrogen separation membrane can beimproved, regardless of the kind of support.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method of preparing a palladium alloy compositemembrane for hydrogen separation, comprising: (a) forming a palladiumcoating layer on a porous support using a dry sputtering depositionprocess; (b) forming a metal coating layer on the palladium coatinglayer using a dry sputtering deposition process; and (c) subjecting themetal coating layer to a reflow process under a hydrogen atmosphere toform an alloy layer.
 2. The method according to claim 1, wherein theporous support is a porous metal support or a porous ceramic support. 3.The method according to claim 2, wherein the porous metal support is aporous nickel support.
 4. The method according to claim 1, wherein themetal coating layer is formed of copper.
 5. The method according toclaim 1, further comprising modifying a surface of the porous supportusing a hydrogen plasma surface treatment before the formation of thepalladium coating layer of the step (a).